Vaccine and Treatment Evaluation Units: The Contractor shall be able to perform identification, design, development, implementation, conduct, analysis, completion, and reporting of clinical studies and clinical trials, including responsibilities to carry out associated tasks and support functions, such as sample analysis, in support of understanding, diagnosing, treating, and preventing infectious diseases caused by viral, bacterial, parasitic and fungal pathogens, including the National Institute of Allergy and Infectious Diseases (NIAID) Priority Biodefense Pathogens and emerging infectious disease pathogens. Opioid use causes a myriad of effects which contribute to significant morbidity and early mortality. Furthermore, the use of opioids is associated with risky sexual behavior and injection drug use (IDU), two major factors in the transmission of HIV, HCV and STDs, with IDU identified as a major form of transmission of HIV and HCV in urban and suburban areas of the United States. Through these high-risk behaviors, persons with opioid use disorder (OUD) develop both direct infectious comorbidities (e.g. blood stream infections and infectious endocarditis), as well as risk-associated infectious illnesses (e.g. sexually transmitted infections, HCV and HBV) which have considerable downstream health care effects. There is a desperate need for better pharmacologic agents in the treatment of OUD that go beyond avoidance of withdrawal to effectively reduce cravings for opioids. Currently, therapeutic interventions for OUD are limited both in options and scale. As such, the development of new therapeutic options for OUD is a critical hurdle in confronting the opioid epidemic. Overall Project aim: explore the therapeutic potential of ANS-6637 to reduce drug craving for the treatment of opioid use disorder (OUD), focusing in a multidisciplinary manner on safety, efficacy, and drug-drug interactions in a sequential phase II-III approach.